[Lapack] lapack bug in divide-and-conquer: is anybody listening

From: Li, Ren-cang
Date: Sat, 20 Nov 2010 12:03:11 -0600

Jim,
Each iteration operates within a shrinking interval which contains the desired
root, and the bisection is called whenever the iteration goes out of the
interval; so the bisection is the fall back (I just double checked). 200 is
extreme, but I think it should never get that far, otherwise I would be really
worried. It would be helpful if someone could tell me what is the exact number
of iterations needed to fix that bug.
best, -rencang
________________________________________
From: James Demmel [demmel@Domain.Removed]
Sent: Saturday, November 20, 2010 10:00 AM
To: julie langou
Cc: lapack@Domain.Removed; lapackers; beboppers; Li, Ren-cang
Subject: Re: [Lapack] lapack bug in divide-and-conquer: is anybody listening
And sorry for also not getting to this sooner. When you say that the
convergence bug in divide-and-conquer was fixed by raising MAXIT
from 40 to 200 in xlaed4 (written by Ren-Cang), then that makes me
think that some flavor of bisection would be a better fall-back when
convergence fails (since the maximum number of iterations could be
more like the number of bits in the floating point word, so 32 or 64).
This is very intricate code (and asking the to-be-hired programmer to fix
it would likely require a very long learning curve) so let me ask Ren-Cang
if he has any opinions.
Thanks,
Jim
julie langou wrote:

Evan,
Sorry for not getting back to you earlier.
We indeed corrected the bug you found, thank you for the patch. It is
released in LAPACK 3.3.0 (released this week).
See http://www.netlib.org/lapack/bugfixedin33.txt.
This bug was labelled bug0064.
If you could confirm that this issue is fixed, I will add a post on
the forum to let the users know.
Regards,
Julie
On Nov 18, 2010, at 8:25 AM, Evan Drumwright wrote: